Caption: Heart bypass surgery. Image 4 of 7. Surgeon checking blood ejection from the heart after coronary artery bypass graft (CABG) surgery. The tubes (centre) attach to a heart-lung machine, which maintains the body's vital functions while the heart is stopped temporarily during surgery. The metal bars either side of the heart are parts of a retractor, used to keep the chest open. CABG is used to treat stenosis (narrowing) of coronary arteries. An artery or vein, taken from the chest or leg is grafted onto the blocked artery, restoring blood flow to the heart. Blocked arteries can cause heart pain (angina) or a myocardial infarction (heart attack).